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point is that an individual fish in a stream is typi- cally very tolerant; tolerant of high temperatures during the summer, tolerant of low oxygen. In fact, most of them don't die until all of the water is gone and the stream dries up. These two things don't match very well. If indeed they're declin- ing and they're so tolerant, why is that? I think the reason is that as we all know, fish need water, so to have a population of these fishes requires that we have water somewhere on the landscape. What we've discovered is that these fish are high- ly dependent on refuge pools that are supplied by ground water and on surface flow connections at certain times of year when they need to move to specific places to spawn or to grow, or actually get back to those refuge pools to make it through dry summers and the ensuing winter when those pools can't reach to the bottom. So it became clear through more than a decade of research that groundwater was going to be the key if we're going to sustain these native fishes and prevent them from being listed as federally threatened or endangered species. Our most recent research focus has been in the Arikaree Basin in Yuma County where towns of Yuma and Ouray are, for example. We're asking the question there for one of these species that the Colorado Division of Wildlife is particularly interested in: What sort of habitat is going to be needed for persistence of these species at the ba- sin level? In a way we were fortunate to have a drought show up so for us as scientists it allowed us to see nature test the limits of the species we were interested in, which is the brassy minnow, although there are a number of other native plains fishes in the basin. So in looking across that landscape we found that there are indeed strong thresholds of stream drying that cause those populations to be extricated, to be lost in certain sections of the basin during that drought. In addition, water used in that basin for agriculture is very important. The center pivot system was invented in Yuma County. That pumping was important but we didn't know much about that when we ended that first study. Subsequently, I found that two of my colleagues affiliated with the Water Center, Deanna Durnford, groundwa- ter hydrologist, and Ramchand Oad, an ag irriga- tion professor, were also interested in that basin. We formed a research consortium and are now studying the links between pumping percent of pivot irrigation groundwater levels that provides fish habitat in the Arikaree River and then how those fish respond in that habitat. We know that in that basin groundwater has declined more than eight meters, about 25 feet, over about 22 percent of the basin, 950 square km, following the beginning of pumping in the early 60s. This pumping is strongly correlated with declines in river flow. We've been at it about a year, the three of us. We have a graduate student each in our lab working on coordinated projects. With respect to groundwater, clearly we want to predict the long term effects of this pumping as well as the additional combination of when you have a drought you need to do more pumping to supply those crops with water. We want to be able to predict those effects on fish habitat and fish. We've learned quite recently, actually just this last month, some preliminary results, although I would caution that these are indeed preliminary, but we really need to under- stand groundwater in two compartments: the high plains aquifer itself, as well as ribbon of river alluvium that supplies the river itself with water. So we are learning that things like ripar- ian trees as well as pumps that are actually in the river alluvium itself - -right along the river -- have strong short term effects on river flow, whereas those pumps on the uplands that are into the high plains aquifer itself potentially could lower the water table enough so that it no longer supplies the alluvium which supplies the river flow. As a group, we are now trying to understand that hydrology and how it feeds the river. If indeed the river dries up completely, and do this even with core habitat for the fish, I would add that we would probably not see those fish recolonize from downstream. Naturally they would have to be put back in with human intervention. we're trying to understand the groundwater 22 ...._W.._